Fix usage of as_offset

[tehrengruber]

The lowering of as_offset in GT4Py was wrong, which was discovered and fixed recently in GridTools/gt4py#1484. Apparently we have some field operators in icon4py that just massaged the code in such a way that the wrong behavior in GT4Py was "canceled" out. However with the fix in GT4Py this broke and and the type inference on ITIR rightfully complained with an error similar to

E               File "/scratch/mch/jenkins/workspace/Spack/spack_PR_2/spack/opt/spack/linux-sles15-zen3/gcc-11.3.0/py-gt4py-1.0.3.9-cj7moh4ujoru35gc7l5ote23jqmtmbng/lib/python3.11/site-packages/gt4py/next/iterator/type_system/type_synthesizer.py", line 44, in <lambda>
E                 self.type_synthesizer = lambda *args, offset_provider: synthesizer(*args)
E                                                                        ^^^^^^^^^^^^^^^^^^
E               File "/scratch/mch/jenkins/workspace/Spack/spack_PR_2/spack/opt/spack/linux-sles15-zen3/gcc-11.3.0/py-gt4py-1.0.3.9-cj7moh4ujoru35gc7l5ote23jqmtmbng/lib/python3.11/site-packages/gt4py/next/iterator/type_system/type_synthesizer.py", line 352, in apply_shift
E                 assert found
E             AssertionError
E             
E           assert 1 == 0
E            +  where 1 = <Result AssertionError()>.exit_code

This PR is meant as a starting point to fix the remaining stencils (e.g. _truly_horizontal_diffusion_nabla_of_theta_over_steep_points looks like a candidate). Note that we can currently not detect these issues in the frontend as the experimental as_offset has a conceptual problem preventing this which can be understood from this snippet:

@field_operator
def foo(inp_field: Field[Vertex, K], index_field: Field[Vertex, K]):
    return inp_field(as_offset(KOff, index_field))

@field_operator
def foo(inp_field: Field[Vertex, K], index_field: Field[Edge, K]):
    # this passes the frontend type check, but is obviously wrong
    return inp_field(as_offset(KOff, index_field))

I very much suspect field operators that use values of the erroneous field operators, e.g. combined (formerly called fused) field operators, and shift those values have produced incorrect results in the past. For the person taking over this PR please be very careful and validate that my changes below.

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